This invention relates generally to circuits used for testing of integrated circuits. More particularly, this invention provides a digitally programmable high voltage driver circuit with digitally programmable slew rate and digitally programmable current limiting.
Programming and testing of certain types of integrated circuits may require application of a wide range of voltages with controlled amounts of currents to the device under test (DUT) that are not provided by typical ATE systems. For example, programmable logic array (PLA) devices that use fusible links or electrically erasable programmable read only memory (EEPROM) cells may require application of voltages in the range of 8 to 15 volts or precise amounts of currents under a specific controlled pattern for programming and testing purposes. The corresponding programming current (I.sub.pp) specified for a particular pin may vary from, for example, 1 mA to 100 mA. During normal operation, the same pin may act as a configurable I/O pin required to source and sink currents with more than an order of magnitude difference in amount. Therefore, the testing of such pins often requires setting variable current limits. Furthermore, due to operation under high voltages, these devices typically employ high-voltage protection circuitry to prevent adverse affects such as latch-up. The high-voltage protection circuitry requires a minimum turn-on time to operate properly. Also, while fast operation requires fast signal rise and fall times (i.e. higher slew rates), preventing overshoot demands slower signal transitions in some cases. Thus, it would be desirable to have voltages with varying and well controlled slew rates applied to the DUT. Preventing latch-up conditions is another reason for providing current limiting capability. The differing I/V requirements for the two different modes of operation (i.e. programming mode and normal operating mode) in programmable logic devices result in complex test patterns.
Existing ATE systems are not designed to support the types of programming and testing requirements mentioned above. A single test pattern that tests for conditions under both programming and normal operating modes can not be applied to the same pin of the DUT without interrupting test execution and reprogramming the variables when switching from one mode to the other. Moreover, typical logic testers provide output voltages in the range of 0 to 5 volts only. Also, current limiting in applications such as those discussed above can be achieved by, for example, using current-limiting resistors at the output of the ATE voltage driver channels. However, such techniques do not provide a satisfactory solution to the problem of ensuring application of controlled amounts of current for a controlled period of time. One problem is due to the fact that the duration of time the programming current, I.sub.pp, is applied to a programming element, such as an antifuse, is typically controlled by the pulse width of the signal applied to the pin. Since the tester has no way of detecting exactly when the antifuse is blown, if the antifuse blows quickly, the programming current may run through the antifuse for a longer period of time than intended. Excessive amounts of currents through a blown antifuse may cause annealing of the antifuse. Furthermore, programmability of the slew rate of signals generated by testers is not a feature that is available in typical ATE. As a result, every time slew rate requirements for a test signal change, the tester variables must be reprogrammed and a different set of passive components (resistors or capacitors) switched in via relays.
The above shortcomings of existing ATE systems have led to use of additional customized hardware as well as cumbersome and inefficient programming and testing algorithms in order to integrate specialized testing requirements into the existing ATE test environment. Manufacturing costs are therefore increased due to additional required test hardware and increased use of tester time.
From the foregoing, it can be appreciated that there is a need for a high voltage driver circuit with programmable current limiting switchable during a continuous test pattern (i.e. "on the fly") that improves the versatility and efficiency of automatic test equipment.